I have just gone through this issue and this solved it for me.
set back to PPPoE
Change get IPv6 address to non-address
Sync v4 account
Prefix delegation enabled
DNS - Auto
Also the deco app is abit buggy for me when doing this, check back to IPv6 when you have saved settings to check it has saved correctly, sometimes i have had to reenter the settings for it to save.
Give that a try
